The present invention concerns an improved mounting apparatus for a length measuring instrument of the scale-and-scanning unit type.
This type of length measurement instrument is used to obtain a high resolution measure of the relative position of two objects; for example, the bed and the slide of a processing or a measuring machine. Such instruments include a scale and a scanning unit mounted to scan the scale. As protection against environmental influences, the scale and scanning unit are often mounted in a stiff elongated housing. Entrainment means coupled to the scanning unit extend through a slit in a face of the housing, between sealing lips that keep the slit closed, and attach the scanning unit to a mounting surface on the processing or measuring machine whose displacement is to be measured. The housing containing the scale and scanning unit is attached rigidly onto the bed or the slide of the machine so that the frictional force created by the entrainment means rubbing on the sealing lips does not produce undesirable shifts of the housing and, with it, of the scale mounted within the housing.
Because the housing is generally made of a material, such as a light metal, whose thermal expansion coefficient differs from that of the material of the mounting surface to which the housing is rigidly attached, temperature fluctuations may cause relative contraction or expansion of the housing with respect to the mounting surface, thereby creating substantial longitudinal forces in the housing. These longitudinal forces can produce undesirable shifts of the housing surface. Such shifts cause the zero point of the scale to shift and may apply distorting tension or compression to the scale itself, leading to measurement errors.
The West German Pat. No. PS 25 05 585 shows a length measurement instrument in which both ends of the housing are attached to a mounting surface by means of flexible mounting elements. Each mounting element consists of an angular bracket which has been weakened by means of a transverse groove to create a flex line. These mounting elements do indeed allow a bending-free attachment of the housing, and thus of the scale, even to uneven mounting surfaces, but they do not eliminate the disadvantages produced by thermal expansion.
U.S. Pat. No. 3,816,002 describes another length measurement instrument whose housing is mounted onto a mounting surface by means of mounting elements that affix one end of the housing rigidly and the other end in a manner that allows for longitudinal movement of the housing. The element affixing the longitudinally moveable end includes a longitudinal oblong hole in which has been installed a spring-loaded bolt. This arrangement does accomodate temperature induced length changes; however, it has the disadvantage that the spring-loaded bolt may create excessive friction between the housing and its mounting surface. Furthermore, inevitable tolerances between the bolt and the oblong groove may result in undesirable oscillations of the instrument. Such oscillations can present a significant problem when the measuring instrument is mounted to a machine tool.